Operations and Significant Accounting Policies - Reclassifications (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2014	Sep. 30, 2014	Dec. 31, 2014
Operating and Maintenance Expenses to Cost of Sales [Member]
Reclassifications [Line Items]
Reclassification Adjustment	$ 16.8	$ 59.2
Operating and Maintenance Expenses to Transmission Services [Member]
Reclassifications [Line Items]
Reclassification Adjustment	11.9	33.2
Operating and Maintenance Expenses to Taxes Other than Income Taxes [Member]
Reclassifications [Line Items]
Reclassification Adjustment	11.4	33.9
Operating Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment	0.0	0.0
Net Income [Member]
Reclassifications [Line Items]
Reclassification Adjustment	0.0	0.0
Net Income Attributable to ALLETE [Member]
Reclassifications [Line Items]
Reclassification Adjustment	$ 0.0	$ 0.0
Property, Plant and Equipment - Net to Goodwill and Intangible Assets - Net [Member]
Reclassifications [Line Items]
Reclassification Adjustment			$ 1.6
Other Non-Current Assets to Goodwill and Intangible Assets - Net [Member]
Reclassifications [Line Items]
Reclassification Adjustment			3.2
Total Assets [Member]
Reclassifications [Line Items]
Reclassification Adjustment			$ 0.0